In a printing process, color correction is conducted prior to printing performed by a printer. The color correction is conducted in order to check finish of printing, for example, 1) the presence or absence of a typographical error or gap, 2) the presence or absence of dust, a flaw, a smear or the like, 3) gradation reproducibility such as brightness or darkness of printed matter, 4) the color reproducibility of the printed matter, 5) the sharpness of the printed matter, and the like.
The color correction is generally conducted on the basis of proof printing performed by a correction printer. In particular, computerization of pre-printing processing has been progressed in recent years, and therefore, there has been prevailed a digital correction printer which can produce proof sheet directly based on digital data by means of an ink jet system or a sublimation thermal ink-transfer system.
In this case, color matching processing is an important technique in which a color reproducing region of proof sheet produced by the correction printer is approximated to that of printed matter to be produced by a printer, thereby producing a visually similar result. However, in many cases, the respective color reproducing regions inherent to the printed matter and the proof sheet are different from each other, and thus, it is very difficult to accurately approximate the color reproducing regions to each other. For example, ink for use in printing performed by a printer is a pigment type; in contrast, ink for use in a correction printer with an ink jet system as a typical digital correction printer is mainly a dye type. In addition, since the ink of a dye type has a color reproducing region greater than that of the ink of a pigment type, the color reproducing region of the correction printer is intentionally narrowed, to be thus approximated to the color reproducing region of the printed matter.
There has been conventionally known a color matching method comprising the steps of: making look-up tables (LUTs) in which respective color reproducing regions of printed matter and proof sheet are measured by a spectrophotometer, and respective image signals are associated with color measurement values; comparing the respective look-up tables of the printed matter and the proof sheet with each other; compressing (Gamut Mapping) the color reproducing region of the proof sheet to the color reproducing region of the printed matter; obtaining a change value with respect to the image signal of the color reproducing region of the proof sheet; and thus, sequentially changing the image signal of the proof sheet to be implemented on the basis of the result.
However, a favorable result is not always produced due to various factors such as an error of the spectrophotometer or a difference in reflectance of each ink. In such a case, it is necessary to perform an appropriate color correction with respect to a partly improper color. In the method using the known LUTs, the change value is calculated while Gamut Mapping is sequentially performed. The change value depends upon combination of the two LUTs, thereby making it difficult to add a color correction value to a local color.
In the meantime, there has been conventionally known a method in which the change value of the proof sheet with respect to the image signal is previously calculated, and then, a reference value table is created. The image signal of the actual proof sheet is changed based on the reference value table. In this method, since the change value is determined, a correction value can be added in accordance with the designation of a local color. However, the interrelationship of the converted values determined in the reference value table has non-linear characteristics, so that the color matching accuracy of the entire change value may be reduced if the color correction is carried out without maintaining the interrelationship. Consequently, it is necessary to wholly or partly correct the reference value table while maintaining the non-linear characteristics.
Furthermore, most of the above-described operation depends upon try-and-error methods to gradually create the reference value table with an enhanced color reproducibility, and therefore, the reference value table per se created through such work is supported by enormous worker-years of effort.
Thus, a problem to be solved by the present invention is to readily achieve color reproducibility with high accuracy between printed matter and proof sheet in color correction.
Moreover, another problem to be solved by the present invention is to, once a reference value table with an enhanced color reproducibility is completed, readily create the completed reference value table based on an original reference value table.